Magnetic coils are employed in a wide variety of different applications, such as transformers, electric motors, relays and as inductive impedances. Such coils are currently manufactured in two ways. The first and most common method of coil manufacture is the wrapping of circular copper wire on a bobbin which is then placed on a magnetic core. The other method that is sometimes used is the wrapping of a rectangular copper strip on a bobbin as a spiral wound coil which is then placed on a magnetic core.
Coils formed by the first method are quite readily fabricated but have numerous disadvantages which are overcome by strip wound coils. Thus, because the rectangular strip fits better or tighter on a bobbin, a larger amount of conductor may be wound on a bobbin and internal losses are reduced. Strip wound coils are easier to tap and have better thermal heat conductivity, as well as a lesser danger of arcing because consecutive turns lay next to each other rather than being displaced so that no large voltage exists between turns.
Both wire wound and strip wound coils require bobbins which are disadvantageous both in coil winding and in coil use, and neither readily admits of any modification once a coil is completed.